Add toSVGRootInlineBox(), and use it as much as we can

It has type checking assertions, and helps to detect wrong type casting.
This cl removes unnecessary local variable as well.

BUG=N/A

Review URL: https://chromiumcodereview.appspot.com/23447021

git-svn-id: svn://svn.chromium.org/blink/trunk@157733 bbb929c8-8fbe-4397-9dbb-9b2b20218538